The present invention relates to the general field of pyrotechnic munitions and more particularly to that of explosive munitions.
A particular subject-matter of the present invention is a safety igniter for a pyrotechnic munition component comprising a structure in the form of a jacket and a solid pyrotechnic charge present in the structure, the said igniter being intended to bring about the combustion without detonation of the pyrotechnic charge when the munition component is subjected to slow cook off.
Stresses of thermal origin, such as kerosene or propellant fires, indirect heatings, can result in the pyrotechnic reaction of the munitions which are subjected to them.
Explosive-comprising munition components, such as missile warheads, bomb casings, penetrators and submarine munitions, can lead to violent blast or detonation reactions because of their high confinement.
To reduce these reactions to an acceptable level, that is to say to a simple combustion without projection of dangerous splinters, the use is known of a composite explosive charge based on inert polymer binders or energetic polymer binders charged with octogen (HMX), hexogen (RDX), nitroguanidine, ammonium perchlorate, triaminotrinitrobenzene (TATB), oxynitrotriazole (ONTA) and/or aluminium, in combination with a system for deconfinement of the structure of the munition.
The deconfinement system can consist of protective caps which can burst at a predefined pressure, which act as safety valve by releasing a discharge surface to the decomposition gases generated by the pyrotechnic reaction. Other techniques exist, such as the use of fusible components, of cutting cords or of incipient fractures.
This safety concept operates perfectly for intense fires of kerosene type. In this case, the very high temperatures are transmitted to the wall of the munition and then to the charge, which reacts by combustion at the structure/explosive interface as soon as the temperature exceeds the self-ignition temperature of the explosive, which is generally between 200xc2x0 C. and 240xc2x0 C. The combustion gases subsequently make their way to the discharge surfaces.
The case of less intense and longer lasting stresses is more complex.
The xe2x80x9cslow cook offxe2x80x9d stress is specified conventionally and consists in subjecting a munition component to heating by a few degrees per hour until it reacts pyrotechnically, which can occur after several tens of hours. These reactions can be very violent as they begin, in some cases, at the core of the pyrotechnic material in a medium which will have the time to decompose by pyrolysis of the binder and beginning of chemical decomposition of the active materials. Core initiations are frequently observed with large-calibre munitions (bombs, penetrators, submarine munitions). They are the consequence of the thermally highly insulating nature of the explosives and of the beginning of exothermic decomposition within the material. The heat given off cannot be discharged towards the outside and leads to an additional internal rise in the temperature which further accelerates the decomposition until the mass reaction. The greater the dimensions, the lower the reaction temperature.
A simple deconfinement system such as those mentioned above is insufficient in this case to limit the overall level of reaction.
It is known, to limit the level of reaction under slow cook off stresses, to insert, in the vicinity of the deconfinement device, a safety igniter which reacts by combustion at a temperature lower than the reaction temperature of the main charge of the pyrotechnic munition, the said combustion of the igniter leading to the combustion without detonation of the main charge.
U.S. Pat. No. 5,786,544 and GB 2 313 653 disclose such safety igniters, composed essentially of a plastic tube comprising an ignition powder or ignition pellets. The igniter is embedded in a ring of foam which separates it from the charge, in the rear part of the munition component, close to deconfinement holes.
The ignition pellets present in the tube are preferably composed of a mixture of boron and of barium chromate. In point of fact, it turns out that barium chromate is particularly toxic and carcinogenic and that it leads to hereditary genetic damage. Furthermore, under thermal stresses, it gives off fumes which are also highly toxic.
Other solutions relating to the nature of the ignition pellets are provided but none is truly satisfactory.
The use of pellets based on nitrocellulose-nitroglycerine double base propellant exhibits, for example, problems of migration of the nitroglycerine on storage, with the pyrotechnic risks which this results in.
There thus exists, for a person skilled in the art, a need for a safety igniter which makes it possible to provide the function described above but which does not exhibit disadvantages, such as those mentioned above.
The present invention provides a solution to this problem and has, as main subject-matter, a novel safety igniter for a pyrotechnic munition component comprising a structure in the form of a jacket and a solid pyrotechnic charge present in the structure, the said igniter being intended to bring about the combustion without detonation of the pyrotechnic charge when the munition component is subjected to slow cook off.
This novel safety igniter according to the invention is characterized in that it comprises a block of solid composition based on pentrite and, preferably, in that it is composed solely of such a block.
Such a safety igniter is particularly simple to manufacture and to insert in the munition component. A single block, it exhibits sufficient intrinsic mechanical properties which do not require an inert housing, such as a plastic tube.
Furthermore, this block of solid composition based on pentrite is a detonatable material contributing additional energy in the context of normal use of the munition, which is not the case with the abovementioned igniters of the state of the art, which are not detonatable. This duality of function, safety igniter in the context of slow cook off and explosive in the context of normal use of the munition, which is specific to the present invention, is particularly advantageous.
It should also be noted that the constituents of the safety igniter according to the invention are neither toxic nor carcinogenic and that the combustion fumes are not particularly toxic.
It is known, in the state of the art, to use solid compositions based on pentrite as priming relay for explosive munitions. Patent Application PCT WO 99/53264 discloses, for example, such a use, which is unrelated to that of safety igniter which is a subject-matter of the present invention. This known priming relay use dissuaded even a person skilled in the art from envisaging the use of these pentrite-based compositions in the function, described above, of safety igniter.
In the context of the present invention, xe2x80x9cslowxe2x80x9d cook off should be understood as cook off of between 0.5xc2x0 C./h and 50xc2x0 C./h, preferably of between 1xc2x0 C./h and 20xc2x0 C./h, even better still between 2xc2x0 C./h and 10xc2x0 C./h, for example approximately 3xc2x0 C./h or 4xc2x0 C./h.
Composition xe2x80x9cbasedxe2x80x9d on pentrite should furthermore be understood as a composition having a content by weight of pentritexe2x89xa75%, better stillxe2x89xa710% and better stillxe2x89xa725%, the maximum content by weight being approximately 98%.
According to a preferred alternative form of the invention, the block of solid pyrotechnic composition based on pentrite is a composite explosive (cast plastic bonded explosive). Composite explosives are generally well known to a person skilled in the art. They are obtained from explosive compositions with a plastic binder processed by casting and then polymerization and are composed of a charged plastic binder comprising at least one nitrated organic explosive charge, such as hexogen, octagen or pentrite. Other oxidizing charges, such as, for example, ammonium perchlorate, or reducing charges, such as, for example, aluminium, can also be present.
More specifically, to prepare the block of composite explosive based on pentrite used as safety igniter according to the invention, first of all the pentrite, and optionally the other explosive or nonexplosive charges, is/are mixed with a liquid polymerizable resin and optionally a plasticizer, and then the paste obtained is cast in a mould with the dimensions desired for the block. The paste is subsequently polymerized. According to the choice and the adjustment of the crosslinking agents, catalysts or wetting agents, composite explosives with varied characteristics are obtained.
The mould can be composed of a cavity machined into the solid pyrotechnic charge of the munition which it is desired to make safer.
According to a preferred alternative form, the plastic binder is a polyurethane binder, the content of which is preferably between 12% and 20% by weight with respect to the total weight of the composite explosive. Preference is given, among polyurethane binders, to those obtained by reaction of a hydroxylated polybutadiene with a polyisocyanate.
Other types of binders can be used, in particular silicone binders and polyester binders.
According to another alternative form of the invention, the block of solid pyrotechnic composition based on pentrite is a compressed explosive, that is to say an explosive with a plastic binder processed by compression. The base material (powder to be moulded) is composed of granules in which the charges are coated with a thermoplastic according to a technique well known to a person skilled in the art.
After reheating the powder to be moulded to a temperature such that the thermoplastic binder begins to soften, it is introduced into a heated mould and then compression is carried out under high pressure, of the order of 103 bar.
According to another alternative form of the invention, the block of solid composition based on pentrite is a melt-cast explosive, for example a pentolite (mixtures of TNT and of pentrite), such as pentolite 20-80 (20% by weight of pentrite and 80% by weight of TNT) and pentolite 50-50. 
Melt-cast explosives, which are well known to a person skilled in the art, are processed by casting in moulds a suspension of a granular explosive in a molten explosive, such as TNT.
According to another alternative form of the invention, the block of solid composition based on pentrite is a pentowax, that is to say a composition composed essentially of pentrite coated with a film of wax, such as beeswax or a synthetic wax.
The method of coating, for example under water, is well known to a person skilled in the art.
The content by weight of wax is preferably between 2% and 12%. Such compositions can also comprise additives, such as graphite and/or aluminium.
The processing of the pentowaxes is carried out by cold compression in the mould of a press.
In the context of the present invention, the block of solid pyrotechnic composition based on pentrite can have any shape.
Preferably, the block is provided in the cylindrical form and better still in the form of a cylinder generated by rotation generally having a diameter of between 2 mm and 50 mm.
The diameter of the block can be less than, equal to or greater than the critical diameter of the solid pyrotechnic composition based on pentrite constituting the block.
The cylinder can have any height. Use is generally made of diameter/height ratios of between 0.5 and 3 but, preferably, this ratio is in the region of 1 or greater than 1.
It has been found, with surprise, that the reaction temperature of the safety igniter, when the munition component is subjected to slow cook off, is a decreasing function of the diameter of the block and that it is thus possible to very easily predetermine the reaction temperature of the safety igniter as a function of the diameter of the block for a given composition and a given diameter/height ratio.
This particularly easy adjustment of the reaction temperature of the safety igniter offers an appreciable advantage in modifying the safety margins of a given munition or in using igniters with the same composition in munitions comprising pyrotechnic charges with different compositions.
Another subject-matter of the present invention is a pyrotechnic munition component comprising a structure in the form of a jacket, generally a metal jacket and for example a steel jacket, a solid pyrotechnic charge present in the structure, a device for deconfinement of the structure, for example a system such as mentioned above, and a safety igniter, also such as mentioned above and a subject-matter of the present invention, which makes it possible to bring about the combustion without detonation of the pyrotechnic charge when the munition component is subjected to slow cook off.
The solid pyrotechnic charge present in the structure is preferably explosive. In this case, the explosive charge is preferably a composite explosive but it can also, for example, be a compressed explosive, a melt-cast explosive, for example based on TNT, or a wax-coated explosive.
The solid pyrotechnic charge present in the structure can sometimes be a propulsive charge, for example a solid propellant, preferably a composite propellant.
Whether the charge is propulsive or explosive, the safety igniter makes it possible, in a slow cook off situation, to bring about the combustion without detonation of the charge but also without propulsion of the munition component, of the structure or of structural fragments.
According to a preferred alternative form of the invention, the safety igniter is situated close to the device for deconfinement of the structure, so as to facilitate the escape of the combustion gases.
According to another preferred alternative form of the invention, the igniter is at least partially embedded in the solid pyrotechnic charge. For this, it is possible, for example, to machine, in the charge, a housing for the igniter. Such a housing can also be produced during the manufacture of the charge by moulding using a removable core. The igniter is subsequently placed in the housing. Adhesive bonding can optionally be carried out in order to promote the maintenance of the igniter in the housing.
It is also possible, and preferable, during the manufacture of the charge by moulding to insert the safety igniter in the explosive paste after it has been cast and before it has been polymerized. After polymerization of the paste, the safety igniter is thus entirely integral with the charge.
The igniter can also not be at least partially embedded in the charge, that is to say be independent of the charge. It can, for example, be held on the structure using conventional bindings or alternatively can be embedded in a foam situated in a chamber inserted for the expansion of the gases close to the deconfinement device.
According to the present invention, when the munition component is subjected to slow cook off and when the temperature reaches the predetermined reaction temperature of the safety igniter, the latter ignites. The hot gases and the particles resulting from the combustion of the igniter then initiate the combustion of the charge of the munition, which burns without detonation or propulsion of the munition component, the structure or structural fragments.
Another subject-matter of the present invention is a process which makes it possible to bring about the combustion without detonation of a solid pyrotechnic charge present in the structure in the form of a jacket of a pyrotechnic munition component when the latter is subjected to slow cook off, the said munition component comprising a device for deconfinement of the structure and a safety igniter as mentioned above according to the invention which, during slow cook off, reacts by simple combustion at a temperature below the reaction temperature of the pyrotechnic charge and then brings about the combustion without detonation of the pyrotechnic charge.